The present invention relates to a laser printer, and more particularly to a color printer which obtains a color image by transferring and superposing different color toner images onto an intermediate transcription member.
A color laser printer forms a color picture by transferring and superposing a plurality of toner images having different colors, which have been formed on a photosensitive member utilizing electro-photographic technology, onto an intermediate transcription member, so as to carry out color picture printing. To accomplish this, a photosensitive member, such as a photosensitive drum or photosensitive belt having photo-conductivity, is rotated with a given speed; the outer surface of the photosensitive member is charged uniformly; a laser beam scanning in a direction perpendicular to the rotating direction of the photosensitive member is intermittently controlled according to video data corresponding to a picture to be printed in order to form an electrostatic latent image on the outer surface of the photosensitive member with exposure control; and a toner image is formed by developing the electrostatic latent image formed on the outer surface of the photosensitive member by said exposure. The printer is constructed so as to repeat the process a number of times wherein the toner image is transcribed and transferred onto an intermediate transcription member rotating in contact with the photosensitive member. Electrostatic latent images for different color toner images are repeatedly formed onto the photosensitive member according to video data received from an image data generator different color images are formed by developing the electrostatic latent images; a color toner image is formed on the intermediate transcription member in such a way that the toner images are repeatedly transcribed and transferred onto the intermediate transcription member; and then, the color toner image is transferred onto paper where the toner is fixed.
In such a laser printer, having a photosensitive member and an intermediate transcription member, the accuracy in positioning each of the different color toner images on the intermediate transcription member is important to obtaining a high definition color image. For this purpose, a printer has been proposed including control means which controls both the driving means for the photosensitive member and for an intermediate transcription member in synchronism with a common basic clock signal and which controls the exposure and transcription timings for each of the colors using the basic clock signal, as described in Japanese Patent Application Laid-Open No. 62-195687 (1987). Further, a color printer is proposed in Japanese Patent Application Laid-Open No. 63-81370 (1988) in which a transcribed paper transmitting belt is rotated by being driven with a plurality of photosensitive drums in contact with the photosensitive drums and the speed of each photosensitive member driving means is controlled by detecting the speed of the transcribed paper transmitting belt.
In such control means for a conventional color printer, the distortion of the photosensitive member and the intermediate transcription member due to changes in environment, such as temperature, is not taken into consideration. Thus, it is difficult, when such a distortion occurs, for the former control unit to ensure that the photosensitive member and the intermediate transcription member will always be driven with a synchronizing speed, since a relative speed difference between the photosensitive member and the intermediate transcription member appears even though they are driven in synchronism with the basic clock signal due to the distortion.
Since either the photosensitive member or the intermediate transcription member is generally formed in a belt-shape, and in most cases the driving means is constructed so as to frictionally transmit the driving force, a relative speed difference is also induced by slip caused in the transmitting portion or by change in the slip with age. In addition to this, in color image printing by superposing a plurality of different color toner images, the apparent outer diameter of the intermediate transcription member changes with the superposing of the toner images onto the intermediate transcription member so as to cause a speed difference as well.
With regard to speed control for the driving means, the timing of the control is not taken into consideration. When a control target value, for example, is changed during the forming of an electrostatic latent image onto the photosensitive member, a disorder appears in the middle of the image being formed so as to cause a positional slip between the image and the electrostatic latent images for the preceding colors.
Furthermore, in the latter construction, where the transcribed paper transmitting belt is driven in contact with a plurality of the photosensitive drums, there has been a problem resulting from inability to obtain a high definition color image caused by shear in the top front positions for each of the color images due to a disorder in synchronization which induces a change in the pitch between rasters when a difference in slip conditions occurs in each of the contact portions.